Towards an ultrasensitive method for the determination of metal impurities in carbon nanotubes.

Residual catalyst metal nanoparticles remain one of the major obstructions in the utilization of carbon nanotubes (CNTs) in many areas owing to their ability to participate in redox chemistry of biomarkers. Presented here is a comparative study of several techniques for quality control of carbon nanotubes in terms of metallic impurities, namely magnetic susceptibility, electron paramagnetic resonance, energy-dispersive X-ray spectrometry, X-ray photoelectron spectroscopy, and thermogravimetric analysis. It is found that the dc magnetic susceptibility is the most sensitive method such that the difference between two CNT samples that underwent slightly different treatments can be detected, whereas the two samples are indistinguishable by other techniques. Therefore, it is suggested that the most accurate statistical method for quality control of carbon nanotubes is dc magnetic susceptibility, which allows the detection of traces of magnetic metal impurities embedded in purified carbon nanotubes, whereas other methods may provide false "impurity-free" information.